Biomedical Engineering Reference
In-Depth Information
7.7
(a, b) TEM images of 5wt% MWCNT-SiO
2
composites. Random
alignment of MWCNTs can be seen in the amorphous glass matrix.
7.5.3 Transmission electron microscopy (TEM)
TEM provides information on the actual state of embedded CNTs in
composites (i.e. their dispersion and orientation), as well as the degree of
crystallinity after processing and the state of the fiber-matrix interface.
Figure 7.7 shows TEM images of CNT-SiO
2
composites revealing
homogeneous dispersion and random orientation of CNTs in a silica glass
matrix.
TEM also reveals the structural characteristics of CNTs before and after
their incorporation in matrices, i.e. possible CNT curvature and damage.
Due to their flexible nature and the intrinsic curvature often associated with
CVD growth, images of bent CNTs are common in composites with glass
(Ning et al., 2004, Thomas et al., 2009, Xu et al., 2009, Zhan et al., 2005,
Zheng et al., 2008) and glass-ceramic (Mukhopadhyay et al., 2010, Otieno
et al., 2010, Ye et al., 2006) matrices. Localized curvatures in the CNT walls
are also observed, which may be due to structural defects produced during
synthesis (Berguiga et al., 2006) or due to the effect of pressure applied
during sintering to densify composites. In addition, folds and kinks have
been noted in the walls of CNTs (Otieno et al., 2010) and curled and
imperfect CNT layered structures are commonly observed (Zhan et al.,
2005).
Another aspect of considerable importance in the characterization of
composites is related to the interface, as the interface characteristics can
significantly affect the composite properties. HRTEM is used to reveal the
type of contact between CNTs and the glass/glass-ceramic matrices.
Mechanical bonding is reported to form predominantly in CNT-glass/
glass-ceramic matrix composites without the existence of an intermediate
phase (Subhani, 2012a). However, chemical bonding at the CNT/matrix
